1. Field of the Invention
The present invention relates to a display device equipped with a screen that displays a character, a still image, a moving image, or the like and relates to a technique to improve visibility of a display screen in various environments where the display device is used.
2. Description of the Related Art
In recent years, a so-called self-luminous type display device having a pixel that is formed of a light-emitting element such as a light-emitting diode (LED) has been attracting attention. As a light-emitting element used for such a self-luminous type display device, an organic light-emitting diode (OLED) (also called an organic EL element, an electro luminescence: EL element, and the like) has been drawing attention and used for an EL display (for example, an organic EL display or the like). Since a light-emitting element such as an OLED is a self-luminous type, it has advantages such as higher visibility of pixels than that of a liquid crystal display, and fast response without requiring a backlight. The luminance of a light-emitting element is controlled by a current value flowing through it.
As a driving method for controlling a light-emitting gray scale of such a display device, there are a digital gray scale method and an analog gray scale method. According to the digital gray scale method, a light-emitting element is turned on/off in a digital manner to express a gray scale. Meanwhile, the analog gray scale method includes a method for controlling the light-emitting intensity of a light-emitting element in an analog manner and a method for controlling the light-emitting time of a light-emitting element in an analog manner.
In the case of the digital gray scale method, there are only two states: a light-emitting state and a non-light-emitting state. Therefore, only two gray scales can be expressed if nothing is done. Accordingly, another method is used in combination to achieve multiple gray scales. A time gray scale method is often used as a method for multiple gray scales (see Reference 1: Japanese Patent Application Laid-Open No. 2001-324958 and Reference 2: Japanese Patent Application Laid-Open No. 2001-343933).
As a display for controlling a display state of a pixel in a digital manner and expressing a gray scale in combination with a time gray scale, there are some displays as well as an organic EL display using the digital gray scale method. As an example, there is a plasma display or the like.
The time gray scale method is a method for expressing a gray scale by controlling the length of a light-emitting period or the frequency of light emission. In other words, one frame period is divided into a plurality of sub-frame periods, each of which is weighted with respect to the frequency of light emission and a light-emitting period, and then the total weight (the sum of the frequency of light emission and the sum of the light-emitting period) is differentiated for each gray scale, thereby expressing a gray scale.
In the meantime, an apparent image quality is emphasized even in such a display panel and many display panels provided with a function to adjust brightness or contrast automatically or manually prevails widely. For example, a liquid crystal display device provided with adjustability to improve visibility of a gray scale by changing transmittance of a liquid crystal without increasing luminance of a backlight of a liquid crystal panel is known (see Reference 3: Japanese Patent Application Laid-Open No. 2003-186455).
However, although a liquid crystal display panel has preferable visibility in an indoor environment of 300 to 700 luxes, there is a problem that the visibility is deteriorated significantly in an outdoor environment of 1,000 luxes or more. There is a liquid crystal panel having a structure where external light is reflected by a pixel electrode, which is referred to as a reflective liquid crystal panel; however, the image quality is rather deteriorated under indoor florescent light; thus, the problem is not solved fundamentally. In other words, it is not solved to ensure visibility in wide range of under dark or indoor florescent light to outdoor sun light.